Communication devices such as User Equipments (UE) are also known as e.g. mobile terminals, wireless terminals and/or mobile stations. User equipments are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
User equipments may further be referred to as mobile telephones, cellular telephones, Machine-to-Machine (M2M) devices, laptops, or surf plates with wireless capability, just to mention some further examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another user equipment or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “base station”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. A cell may be used as a Primary Cell (PCell and a Secondary Cell (SCell) by a UE, for different carrier aggregation deployments and scenarios see 3GPP 36 300 Annex J. PCells and SCells will be discussed more in detail below. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even base stations, may be directly connected to one or more core networks. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. Data transmission is in LTE controlled by the radio base station.
LTE uses Orthogonal Frequency Division Multiplex (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource may thus be seen as a time-frequency grid where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.
Furthermore, the resource allocation in LTE is typically described in terms of Resource Blocks (RB), where a resource block corresponds to one slot, 0.5 ms, in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction, 1.0 ms is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of Virtual Resource Blocks (VRB) and Physical Resource Blocks (PRB) has been introduced in LTE. The actual resource allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain, thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled, i.e. in each subframe the base station transmits control information about to which user equipments data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also comprises Common Reference Symbols (CRS), which are known to the receiver and used for coherent demodulation of e.g. the control information.
Carrier Aggregation
The LTE Rel-10 specifications have recently been standardized, supporting Component Carrier (CC) bandwidths up to 20 MHz, which is the maximal LTE Rel-8 carrier bandwidth. Hence, an LTE Rel-10 operation wider than 20 MHz is possible and appear as a number of LTE carriers to LTE Rel-10 user equipment.
In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable user equipments compared to many LTE legacy user equipments. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy user equipments, i.e. that it is possible to implement carriers where legacy user equipments can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 user equipment can receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier.
The Rel-10 standard support up to five aggregated carriers where each carrier is limited in the 3GPP Radio Frequency (RF) specifications to have a one of six bandwidths namely 6, 15, 25, 50, 75 or 100 RB, corresponding to 1.4, 3 5 10 15 and 20 MHz respectively.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in the network may be different from the number of CCs seen by a user equipment: A user equipment may for example support more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs.
During initial access a LTE Rel-10 user equipment behaves similar to a LTE Rel-8 user equipment. Upon successful connection to the network a user equipment may, depending on its own capabilities and the network, be configured with additional CCs in the UL and DL. Configuration is based on Radio Resource Control (RRC). The Radio Resource Control (RRC) protocol belongs to the UMTS Wideband Code Division Multiple Access, (WCDMA) protocol stack and handles the control plane signalling of Layer 3 between the UEs (User Equipment) and the UTRAN. Due to the heavy signaling and rather slow speed of RRC signaling it is envisioned that a user equipment may be configured with multiple CCs even though not all of them are currently used. If a user equipment is activated on multiple CCs this would imply it has to monitor all DL CCs for the Physical Downlink Control Channel (PDCCH) and the Physical Downlink Shared Channel (PDSCH). This implies a wider receiver bandwidth, higher sampling rates, etc. resulting in high power consumption.
Component Carrier Types
Initially, the user equipment will be configured with one UL/DL pair of component carriers, on which it made the initial random access. These component carriers are together called the Primary Cell (PCell). In addition to the PCell, the base station may configure the user equipment with additional serving cells, so called Secondary Cells (SCells) as extra resource when needed.
The UL PCell is configured with Physical Uplink Control Channel (PUCCH) and used for transmission of Layer 1 (L1) uplink control information. This also includes Channel State Information (CSI) for the DL transmission on the activated SCells.
The PCell cannot be deactivated. Non-Access Stratus (NAS) information is taken from the PCell. When the DL PCC experiences Radio Link Failure (RLF), Re-establishment of the UEs RRC connection will be triggered, regardless of the RLF status on the other DL CCs.
An SCell may be configured with a downlink and optionally an uplink. The UE may use only downlink and is therefore only optionally configured with an uplink. Thus, from user equipment point of view, the PCell is a UL/DL pair of component carriers, while the SCell may be one DL and optionally an UL. From a base station point of view a cell have an UL and a DL. But if a UE is configured with an SCell with a DL only, the UE will only use the DL of the cell even though other UEs using the same cell of the same base station may utilize both the UL and the DL of the cell.
The SCells are per default deactivated when added, but may be activated and deactivated. This activation/deactivation mechanism is implemented in the Medium Access Control (MAC) layer and can be applied to one or more SCells at the time.
When a downlink SCell is not active, the user equipment does not need to receive the corresponding PDCCH or PDSCH, nor is it required to perform Channel Quality Indication (CQI) measurements. It is also not allowed to perform any UL transmissions.
The user equipment may be configured with one or more, up to four SCells.
Random Access
In LTE, as in any communication system, a user equipment may need to contact the network via the base station without having a dedicated resource in the Uplink. To handle this, a Random Access (RA) procedure is available where a user equipment that does not have a dedicated UL resource may transmit a signal to the base station. The first message of this procedure is typically transmitted on a special resource reserved for random access, a Physical Random Access Channel (PRACH). This channel may for instance be limited in time and/or frequency, as in LTE. The resources available for PRACH transmission is provided to the user equipments as part of the broadcasted system information, or as part of dedicated RRC signaling in case of e.g. handover.
In LTE, the random access procedure may be used for a number of different reasons. Among these reasons are                Initial access, for UEs in the LTE_IDLE or LTE_DETACHED states        Incoming handover        Resynchronization of the UL        Scheduling request, for a user equipment that is not allocated any other resource for contacting the base station        Positioning        
In the contention-based random access procedure used in LTE the user equipment starts the random access procedure by randomly selecting one of the preambles available for contention-based random access. The user equipment then transmits the selected random access preamble on the PRACH to the base station in RAN, this message is sometimes referred to as MSG1. MSG1 is short for “random access message 1” which is the message in which the random access preamble is transmitted.
The RAN acknowledges any preamble it detects by transmitting a random access response referred to as MSG2, including an initial grant to be used on the uplink shared channel, a temporary Cell (C)-Radio Network Temporary Identity (RNTI), and a Time Alignment (TA) update based on the timing offset of the preamble measured by the base station on the PRACH. The MSG2 is transmitted in the DL to the UE and its corresponding PDCCH message CRC is scrambled with the RA-RNTI. RNTI values are used primarily by the base station Physical Layer for scrambling the coded bits in each of the code words to be transmitted on the physical channel. The different RNTI values used to identify the UE are defined in 3GPP TS 36.300, section 8.1.
When receiving the response, the user equipment uses the grant to transmit a message referred to as MSG3 that in part is used to trigger the establishment of radio resource control and in part to uniquely identify the user equipment on the common channels of the cell. The timing alignment command provided in the random access response is applied in the UL transmission in MSG3. The base station may change the resources blocks that are assigned for a MSG3 transmission by sending an UL grant that's CRC is scrambled with a Temporary (T) C-RNTI. The MSG4 which is then contention resolution has its PDCCH CRC scrambled with the C-RNTI if the user equipment previously has a C-RNTI assigned. If the user equipment does not have a C-RNTI previously assigned its PDCCH CRC is scrambled with the TC-RNTI.
The procedure ends with RAN solving any preamble contention that may have occurred for the case that multiple user equipments transmitted the same preamble at the same time. This may occur since each user equipment randomly selects when to transmit and which preamble to use. If multiple user equipments select the same preamble for the transmission on Random Access Channel (RACH), there will be contention between these user equipments that needs to be resolved through the contention resolution message referred to as MSG4. An example of the case when contention occurs is where two user equipments transmit the same preamble, p5, at the same time. A third user equipment also transmits at the same RACH, but since it transmits with a different preamble, p1, there is no contention between this user equipment and the other two user equipments.
The user equipment may also perform non-contention based random access. A non-contention based random access or contention free random access may e.g. be initiated by the base station to get the user equipment to achieve synchronisation in UL. The base station initiates a non-contention based random access either by sending a PDCCH order or indicating it in an RRC message. The later of the two is used in case of handover.
The base station may also order the user equipment through a PDCCH message to perform a contention based random access. In the procedure for the user equipment to perform contention free random access, the MSG2 is transmitted in the DL to the user equipment and its corresponding PDCCH message CRC is scrambled with the RA-RNTI, similar to the contention based random access. The user equipment considers the contention resolution successfully completed after it has received MSG2 successfully.
For the contention free random access as for the contention based random access, the MSG2 does contain a timing alignment value. This enables the base station to set the initial/updated timing according to the user equipments transmitted preamble.
In LTE Rel-10, the random access procedure is limited to the primary cell only. This implies that the user equipment may only send a preamble on the primary cell. Further MSG2 and MSG3 is only received and transmitted on the primary cell. MSG4 may however in Rel-10 be transmitted on any DL cell.
In LTE Rel-11, the current assumption is that the random access procedure will be supported also on secondary cells, at least for the user equipments supporting LTE Rel-11 carrier aggregation. Only network initiated random access on SCells is assumed.
Random Access Failure in Rel-10 and Earlier Releases
When random access fails continuously on the PCell, the maximum number of expected transmission attempts of MSG1, which is the transmission of the Random Access (RA) preamble on PCRACH, preambleTransMax, will be exceeded. When the preambleTransMax threshold is exceeded, the user equipment will indicate a random access problem to higher layers. This will lead to that the user equipment declares radio link failure on the PCell. preambleTransMax is defined in TS 3GPP 36.331 as part of the RACH-ConfigCommon IE as follows: “Maximum number of preamble transmission in TS 36.321. Value is an integer. Value n3 corresponds to 3, n4 corresponds to 4 and so on.”
For contention free random access this is the maximum number of retransmissions using the assigned preamble. For the contention based random access this is the maximum number of retransmissions where for each retransmission a preamble is selected according to the preamble selection method specified in 3GPP TS 36.321.
preambleTransMax may be exceeded either at reception of Random Access Response, MSG2, or at reception of the contention resolution message, MSG4. The procedural details for random access failure on the PCell are described in 3GPP 36.321, chapter 5.1.4 and 5.1.5.
A UE is currently prevented from performing an infinite number of random access re-attempts by triggering Radio Link Failure (RLF).
For SCells there is currently no radio link monitoring and the UE can thus not declare radio link failure on an SCell. Declaring RLF on the PCell as a result of continuous random access failure on an SCell does not seem like a good solution. And introducing RLF/RLM on SCells would mean introducing higher complexity and more error cases.
Hence it seems that the method which is currently applied for the PCell to prevent random access failure from continuing infinitely is not suitable also for SCells. Also, no other known method for preventing an infinite number of RA attempts on the SCell exists.